In annual reports filed between 1997 and 2000 on behalf of his private law firm, Horne denied ever having been a partner in a business that went bankrupt. The truth of the matter, however, was that Horne was the president of T.C. Horne & Co., an investment firm created in the late-1960s that went bankrupt in 1970. Three years later, the future attorney general was sanctioned by the Securities and Exchange Commission (SEC) charging that he had "willfully aided and abetted" his firm in violating securities laws by submitting false balance sheets and misrepresenting his firm's assets. He was eventually barred for life from associating with brokers, dealers, investment advisers, and investment companies. Horne's Republican primary challenger, Andrew Thomas, contended that "the circumstances surrounding the bankruptcy should disqualify Horne from holding the state's top law-enforcement job."

In addition to receiving six speeding tickets from law enforcement officials over an eighteen month period, including one in a school zone, Horne was issued a criminal citation for violating A.R.S. 28-701.02 A2 on Sunday, October 21, 2007 by the Scottsdale Police Department. The charge came with a maximum possible sentence of six months in jail, three years probation, and a $2,500 fine. Three months later, Horne's lawyer was able to negotiate a plea bargain with the Scottsdale prosecutor, who agreed to drop the criminal traffic offense charge in exchange for Horne pleading guilty to a civil offense for violating A.R.S. 28-701A, a charge that normally applies to drivers going 16 to 20 miles per hour above the speed limit; Horne was cited for going 27 miles per hour over the 45 mph speed limit.